The present invention generally relates to methods and systems for determining bearing, and more specifically, for determining bearing independent of frequency.
Today, systems exist for use in aircraft surveillance for collision avoidance and traffic alert. These conventional systems use active interrogation of mode select (Mode-S) and Air-Traffic Control Radar Beacon System (ATCRBS) transponders that incorporate a passive phased array antenna. Conventional Mode-S and ATCRBS transponders transmit encoded messages containing information about the aircraft in response to interrogation signals received from ground based radar or from an aircraft with a traffic avoidance system (TAS), or traffic alert and collision avoidance system (TCAS). When the transponder is not broadcasting, the transponder monitors for transmissions including interrogation signals.
TAS and TCAS equipment transmits interrogation signals that are received and replied to by other aircraft and used to determine the location of other aircraft relative to the originating aircraft position. Conventional TAS and TCAS systems include a 4-element antenna coupled to a radio frequency (RF) transmitter/receiver. The transmitter and receiver are remotely coupled to the antenna array by coaxial transmission lines. The coaxial transmission lines may be several feet in length (e.g., 30 feet long). The antenna array utilized by conventional TCAS systems are “passive”, in that substantially all of the power utilized to drive the antenna array elements is produced at the transmitter/receiver. Similarly, all of the power that is used to boost the receive range of the antenna array is provided at the transmitter/receiver.
The transmitter and receiver are in turn coupled to a signal processor that controls transmission and reception of TAS and TCAS related information and that performs aircraft surveillance operations, such as traffic alert and collision avoidance operations. The transmitter is coupled to the signal processor for transmitting, among other things, interrogation signals. A control panel and display are joined to the signal processor for operating the TAS/TCAS system and for displaying TAS/TCAS information.
The TAS/TCAS system identifies the location and tracks the progress of a target aircraft equipped with beacon transponders. As part of the locating and tracking operations, the TAS/TCAS system determines a bearing to the target aircraft based on received signals from the transponder of the target aircraft. Currently, there are three versions of the TCAS system in use or in development; TCAS I, II, and III. TCAS I, the simplest of the systems, is less expensive but also less capable than the others. The TCAS I transmitter sends signals and interrogates Mode-C transponders. The TCAS I receiver and display indicate approximate bearing and relative altitude of all aircraft within the selected range (e.g., about forty miles). Further, the TCAS system uses color coded dots to indicate which aircraft in the area pose a potential threat (e.g., potential intruder aircraft). The dots are referred to as a Traffic Advisory (TA). When a pilot receives a TA, the pilot then visually identifies the intruder aircraft and is allowed to deviate up to 300 feet vertically. Lateral deviation is generally not authorized. In instrument conditions, the pilot notifies air traffic control for assistance in resolving conflicts.
The TCAS II system offers all of the benefits of the TCAS I system, but also issues a Resolution Advisory (RA) to the pilot. In the RA, the intruder target is plotted and the TCAS II system determines whether the intruder aircraft is climbing, diving, or in straight and level flight. Once this is determined, the TCAS II system advises the pilot to execute an evasive maneuver that will resolve the conflict with the intruder aircraft. Preventive RAs instruct the pilot not to change altitude or heading to avoid a potential conflict. Positive RAs instruct the pilot to climb or descend at a predetermined rate of 2500 feet per minute to avoid a conflict. TCAS II is capable of interrogating Mode-C and Mode-S. In the case of both aircraft having Mode-S interrogation capability, the TCAS II systems communicate with one another and issue de-conflicted RAs.
The TCAS III system is similar to the TCAS II, but allows pilots who receive RAs to execute lateral deviations to evade intruders. The TCAS III system is more accurate and has a smaller bearing error. Another upgrade, that is proposed, has been to add the capability to transmit the aircraft's GPS position and velocity vector to other TCAS-equipped aircraft thus providing information that is much more accurate.
Each of the above-described surveillance systems utilizes a phase antenna array that, during transmission operations, performs antenna pattern phasing and, during reception operations, determines bearing angle to target aircraft. In certain conventional systems, a quadrature demodulator is used as a phase detector circuit to determine the bearing to the target aircraft. Often, the phase detector circuit is formed from numerous analog discrete components that need to be “linearized” through an analog feed-back loop. The feed-back loop complicates system calibration and the overall circuit design. Also, the analog phase detector circuit exhibits a dynamic range that is unduly limited. Further, the phase detector circuits require isolation of each antenna element channel.
Further, a conventional digital phase detector has been proposed that performs direct intermediate frequency sampling through A/D converters. The digital phase detector is used to determine the phases of the received signals. The conventional digital phase detector uses an internal reference waveform that is mixed separately with each of the received signals. The combinations from the mixing operations are then used to calculate an actual phase of each receive signal. However, the conventional digital phase detector has experienced certain limitations. First, calibration of the detector is difficult and complicated. Also, each channel must be isolated to ensure the accuracy of each calculated actual phase.
Improved bearing detection methods and systems are needed that address and overcome the difficulties noted above and otherwise experienced heretofore.